The present invention concerns safety supports for vehicle tires that are mounted inside the tires on the wheel rims in order to bear the load in case of tire failure or abnormally low pressure.
Patent application EP 0,796,747 A1 discloses a safety support made essentially of a resilient elastomeric material having a generally cylindrical base, a generally cylindrical crown and an annular body connecting the base and the crown. The annular body includes, on the side intended to be placed toward the outside of the vehicle, a plurality of recesses extending generally axially to at least the middle of the body without crossing it. The annular body is thus designed to bear the load on compression in its solid part and on buckling in its recessed part.
Although the foregoing safety support affords numerous advantages, excellent shock resistance and a very good lifetime under load, and provides a remarkable behavior for a vehicle running with at least one of its tires bearing on its safety support, it is relatively heavy.
Patent application JP 3-82601 proposes, notably, a safety support intended to be mounted on the rim inside a vehicular tire in order to support the tread of that tire in case of loss of inflation pressure. That safety support includes a generally cylindrical base intended to fit around the rim, a generally cylindrical crown intended to come in contact with the tread in case of pressure loss, and leaving a clearance from the latter at nominal pressure, and an annular body connecting the base and the crown. The annular body contains a circumferentially continuous support member having a circumferential median plane. The support member includes a plurality of partitions extending axially on both sides of the circumferential median plane and distributed about the circumference of the support, and connecting members extending generally circumferentially and joining the two ends on the same side of the support of two adjacent partitions, such connecting members being successively arranged alternately on both sides of the partitions. The partitions and connecting members are generally rectilinear. The difference between the maximum and minimum values of the area of an axial section of the support member as a function of the azimuth, related to the sum of those same areas, is preferably less than 0.3. Consequently, as a function of the azimuth, the area of an axial section of the support member varies at most by a factor of 2 in order to provide good load capacity uniformity and to limit the vibrations on running flat. The support of application JP3-82601 is made essentially of a hard polymeric material and the whole support member is designed to bear the compression load.
The object of the invention is to provide a safety support which, while affording a comparable lifetime, provides improved performance relative to the prior art in respect of the weight of the support.
The safety support according to the invention is intended to be mounted on the wheel rim inside a tire equipping a vehicle in order to support the tread of the tire in case of loss of inflation pressure. The safety support includes a generally cylindrical base intended to fit around the rim, a generally cylindrical crown intended to come in contact with the tread in case of pressure loss, and leaving a clearance from the latter at nominal pressure, and an annular body connecting the base and the crown. The annular body contains a circumferentially continuous support member having a circumferential median plane, which support member has a plurality of partitions extending axially on both sides of the circumferential median plane and distributed about the circumference of the support, and connecting members extending generally circumferentially and joining the two laterally adjacent ends (on the same side of the support) of two circumferentially adjacent partitions, such connecting members being successively arranged alternately on both sides of the partitions. In accordance with the invention, the ratio of the thickness of the partitions in the central part H thereof to the thickness of their lateral ends h is greater than 1.1, and preferably greater than 1.5, in order to reinforce the buckling resistance under a radial load on the annular body.
The invention also concerns a similar support in which the partitions present, from one lateral end to the other, at least one reversal of direction of their curvature in order to reinforce the buckling resistance under a radial load of the annular body.
More specifically, the central part of each partition of the support member is separated from the connecting members and can be destroyed in the course of running flat upon the occurrence of a repeated buckling strain. In the case of supports made essentially of an elastomeric material, the occurrence of such repeated buckling strain during running results in the initiation and propagation of fissures in the side of the extended walls of the support. On the other hand, in the case of supports made essentially of plastics, a buckling strain results in the appearance of plastic deformations. Those irreversible deformations considerably reduce the strength of the structure and its load-bearing capacity, and progressively render it unfit to fulfill its function.
According to the first embodiment of the invention, the ratio between the thickness of the partitions in the center part H thereof to the thickness in their lateral ends h is greater than 1.1 and preferably greater than 1.5. This variation of thickness very appreciably strengthens the buckling resistance of the center part of the partitions, and thus makes it possible, at a given radial load, to limit the thickness of the connecting members and to lighten the total weight of the support.
According to the second embodiment, the partitions include, from one lateral end thereof to the other, at least one reversal, and preferably three reversals, of direction of their curvature.
According to one working example, the partitions include a central part extending generally axially between two lateral parts, which lateral parts join the connecting members by forming an angle xcex3 with the circumferential direction ranging between 20 and 40 degrees.
According to another working example, the partitions present, in their central zone, two generally axially extending parts that are circumferentially offset from each other, as well as a third connecting part. The mean orientation variation a between that third connecting part and the two parts of generally axial orientation is preferably greater than 20 degrees.
Each connecting member can be backed by at least one wall extending generally axially toward the outside of the annular body. Such wall or walls can be placed on just one side or on both sides of the support member. Because these axial walls are integral with the support member and are relatively short, they are not very sensitive to buckling. Such axial walls make it possible, at an equal support width, to reduce the width of the support member and, therefore, increase its buckling resistance.
In a preferred embodiment, each connecting member forms, with an axial wall backing it and the lateral ends of the two adjacent partitions, a star-shaped assembly with three branches. Preferably, the axial width of the axial backing wall is less than or equal to half the axial width of the two adjacent partitions of the support member.
The support members, according to the invention, can also contain a generally cylindrical web coaxial with the support and placed radially, for example, at mid-height, of the support member. The web is made of the same material as the rest of the annular body. When placed at mid-height, the web divides the height of the partitions in two and thus increases the limiting buckling load by a factor of four.
To facilitate construction of the supports according to the invention, the different geometries of the support members are configured to exclude any undercut part that would impede axial stripping of the support.
The supports according to the invention can be made essentially of very different materials.
According to a first example, a natural or synthetic rubber-based compound may be used. In that case, the modulus of elasticity of the compound preferably ranges between 8 and 30 MPa. The modulus of elasticity is understood to be a secant modulus of extension obtained on a deformation on the order of 10% at ambient temperature in the third stress cycle. A support made with such materials and the above-described geometries according to the invention can be half the weight of a support such as described in application EP 0,796,747 A1, while maintaining completely acceptable behavioral performances and running lifetime.
According to a second example, an elastomeric polyurethane can be used. An advantage of that material is that it affords a higher moduli of elasticity, ranging, for example, between 20 and 150 MPa, with limited self-heating on running. By comparison with the support of application EP 0,796,747 A1, that makes it possible to obtain a weight reduction factor greater than 3, while maintaining excellent shock resistance by reason of high breaking elongations, e.g., greater than 100%.
Thermoplastic polymers can also be used, preferably without rigid reinforcing fillers such as balls or fibers. Although such reinforcing fillers make it possible to increase the rigidity of those materials appreciably, they generally reduce the shock resistance unacceptably for good performance of a safety support. The Izod test toughness of such materials at xe2x88x9230xc2x0 C. is preferably greater than 10 kJ/m2 and the breaking elongation is preferably greater than 50%. For example, a polyamide doped with elastomeric particles (such as ZYTEL ST 801), a thermoplastic elastomer (TPE) or a thermoplastic polyurethane (TPU) can be used.
Another material advantageous for supports according to the invention is a polyurethane containing rigid reinforcing fillers such as balls or fibers. Such material makes it possible to attain moduli of elasticity higher than those of the elastomeric polyurethanes, while maintaining acceptable shock properties.